Gas emissions from coal-burning utilities and waste incinerators are the subject of increasing levels of contaminant regulation. Pneumatic conveying of sorbent material is a common method of introducing sorbent into contaminated gas streams. It operates by creating a pressure differential along a pipeline and using the air that moves toward the area of lower pressure to move bulk sorbent material. The process can be performed with a vacuum inducer or by injecting compressed air into one end of or along a pipeline.
The two main categories of pneumatic conveying technologies are low-pressure (dilute-phase) and high-pressure (dense-phase) systems. The former category uses either positive or negative pressure to push or pull sorbent material through the convey line at relatively high velocities at or above the material saltation velocity (e.g., typically above about 15 m/s). It is a high-velocity system that employs a high air-to-material ratio. The second category uses positive pressure to push materials through the convey line at relatively low velocities below the material saltation velocity (e.g., typically no more than about 15 m/s). It is a high-pressure, low-velocity system because it has a low air-to-material ratio.
Dry sorbent injection is increasingly being applied to coal-fired power plant flue gas to comply with new air pollution requirements for toxic air pollutants, including mercury, sulfur trioxide, nitrogen oxides, acid gases (such as HCl), and selenium, and to provide supplemental control for criteria pollutants, such as sulfur dioxide. Systems for injecting powdered or finely milled sorbents for emissions control at coal fired power plants are typically delivered in high sorbent particle velocity and low pressure (typically less than 1 bar g) Dilute Phase pneumatic conveying systems with sorbents entrained in a gas stream at speeds above the sorbent saltation velocity. Dense phase conveying has not previously been applied to sorbent injection systems for coal-fired power plants, but could have important advantages over low pressure injection systems. Sorbents used for emission control at coal-fired power plants may include, for example, powdered activated carbon (PAC) for mercury control, dry sorbent injection (DSI) including powdered trona, lime and sodium bicarbonate for acid gas control and other finely divided materials that are injected into the flue gas to adsorb or absorb or otherwise react with specific pollutants.